The present invention relates to polyoxymethylene resin compositions. Molded articles of the present polyoxymethylene resin composition are suitable for use in parts of precision devices, household electrical appliances, office automation (OA) appliances, automobiles, industrial materials, sundry goods, etc.
Polyoxymethylene resins have been widely used as engineering resins having balanced mechanical properties and distinguished wear resistance and thus have been widely used not only in various mechanical working components, but also in OA appliances, etc. However, conventional polyoxymethylene resins do not have satisfactory impact resistance.
Therefore, attempts have been made to obtain resin compositions by blending the polyoxymethylene resin compositions with elastomer components. For example, U.S. Pat. Nos. 4,804,716 and 4,978,725 disclose the art of blending polyoxymethylene resins with polyurethane resins. U.S. Pat. No. 4,277,577 discloses the art of blending polyoxymethylene resins with olefinic elastomers and polyurethanes. U.S. Pat. No. 4,665,126 discloses the art of blending polyacetal resins with multilayered interpolymers and thermoplastic polyurethanes. EP-A-290761 discloses the art of blending polyoxymethylene resins with thermoplastic polyurethanes and polyether block copolyamides. JP-A-8-183115 discloses the art of blending polyacetal resins with polyalkylene oxides.
In these disclosures, the art of blending polyoxymethylene resin with polyurethane is the most practical, but the resulting polyurethane resins have such problems as (1) higher cost, (2) poor slidability, (3) poor strength at the welded parts of molded articles, etc.
It has been also proposed to blend polyoxymethylene resins with polyamide elastomer such as are used in the present invention. For example, JP-A-59-191752 discloses resin compositions with good anti-static properties, prepared by blending polyacetal resins with polyether ester amides. JP-A-63-110245 discloses resin compositions with good impact resistance and anti-static properties, composed of polyacetal resins and polyether ester amides. U.S. Pat. No. 5,043,399 discloses resin compositions with good impact resistance and anti-static properties, composed of polyacetal resins and polyether ester amides. JP-A-4-168145 discloses resin composition with good mechanical properties, anti-static properties and frictional wear resistance, composed of polyacetal resins, polyether ester amides and lubricants. JP-A-4-370156 discloses resin compositions with good antistatic properties and impact resistance, composed of polyacetal resin-containing thermoplastic resins and modified polyamide elastomers prepared by graft polymerization of polyamide elastomers selected from polyether amides, polyether ester amides and polyester amides with 1,2-alkylene oxides.
However, these resin compositions are considerably poorer in impact resistance as compared with resin compositions containing the above-mentioned polyurethanes, and also do not have a satisfactory level of frictional wear resistance.
As for the art of using acid-modified olefinic resins as used in the present invention, U.S. Pat. No. 4,556,690 discloses resin compositions composed of polyacetal resins and acid-modified α-olefin copolymers. However, polyacetal resins are not resistant to acids, so that the polyacetal resin is sometimes decomposed when kneaded at elevated temperatures under high shearing conditions, and the impact resistance of the resulting resin compositions is quite poor, as compared with that of the above-mentioned resin compositions prepared by blending polyoxymethylene resins with polyurethanes.
Also in the art of using polyamide elastomers and acid-modified olefin copolymers, U.S. Pat. No. 5,652,326 discloses resin compositions comprising polyolefinic resins, polyether ester amides, polyester resins, and/or polyacetal resins, and modified low molecular weight polyolefins. However, the resin compositions disclosed therein are basically dispersions of other components in the polyolefinic resins, where the object of adding polyacetal resins thereto is to facilitate exudation of polyether ester amides to the resin surface.